1. Field of the Invention
The present invention relates to a method for supplying a hydraulically-operated device with a working medium, as well as to a control system for supplying a hydraulically-operated device with a working medium.
2. Description of the Related Art
It is known to supply a hydraulically-operated device with a working medium, wherein the pressure of the working medium can be adjusted as a function of a control variable through pressure limiting or pressure reduction. For that purpose, conventional pressure limiting or pressure reduction valves are provided that can be controlled by a control means, for example a pilot-controlled proportional valve. In order to allow the hydraulic device to work at various operating points, the working medium pressure that is adjusted by pressure limiting or pressure reduction can be varied within a nominal pressure range. For a hydraulic device on which external forces influence the power output, a feature is provided to allow the working medium pressure to be adjusted beyond the nominal pressure range for certain external force values. For example, for hydraulic adjusting devices it can be necessary to increase the working pressure range beyond the nominal pressure range in order to achieve the desired adjustment result. Thus, the working medium pressure can also be adjusted in a maximum pressure range, wherein that maximum pressure range is between the nominal pressure range and a system-related maximum pressure.
The adjusted working medium pressure acting on the hydraulic device is, as mentioned above, a function of the control variable. That means that in the case of control with an electrical control variable, the working medium pressure rises if the control variable, especially an electric current, increases as well. Of course, the opposite can also be provided, so that the working medium pressure rises with decreasing electric current. There is therefore a proportionality between the control variable and the working medium pressure. When that relationship is plotted in a coordinate system, with the control variable being entered on one of the coordinate axes and the working medium pressure on the other coordinate axis, the result is a nearly straight characteristic line with a constant slope across the entire control variable range.
A disadvantage is that with the application of greater pressure intensification, because the characteristic line has the same slope over the entire nominal pressure range and the entire maximum pressure range, it therefore proceeds relatively steeply. That is particularly the case when the upper pressure limits in the nominal and maximum pressure ranges are far apart. Due to the high pressure intensification, a relatively small change in the control variable therefore effects a very large change in the working medium pressure for the hydraulically-operated device. That is particularly disadvantageous in the nominal pressure range, because the working medium pressure should be adjusted relatively precisely to the hydraulically-operated device.
For example, in automatic transmissions with stepless gear ratio means, particularly belt-driven, conical pulley transmissions, the conventional method for controlling the pressure of the conical disks against the endless, torque-transmitting member is disadvantageous, because in the nominal pressure range the application pressure cannot be adjusted very precisely. The application pressure must be adjusted precisely as a function of the torque to be transmitted. Imprecise adjustment particularly affects the efficiency of the transmission, because too high an application pressure impairs efficiency. In the case of too low an application pressure, there is a risk that the endless, torque-transmitting member will slip and will damage the conical disks. The high pressure intensification in the known method or the known control system affects the overall hysteresis of the control system, wherein the overall hysteresis is composed of the hysteresis of the control means and of the pressure limiting or pressure reduction valve.
That means that the overall hysteresis is composed of the hysteresis of the control means multiplied by the pressure intensification and the valve body member hysteresis force, divided by the pressure feedback area, which should be regarded as the quotient of the pilot surface area and the pressure intensification. Pressure intensification as a factor, therefore, has a great influence on the overall hysteresis.
The purpose of the invention, therefore, is to provide a method and a control system for supplying a hydraulically-operated device with a working medium, and that do not produce the above-described disadvantages.